Resonant tuned vibratory material handling equipment such as feeders, screens, vibratory conveyors and vibratory parts feeders move bulk material and small parts by vibrating a tray or bed on which the material rests. The vibratory material handling equipment moves the bulk material or small parts back-and-forth at a predetermined frequency, a predetermined stroke and a preselected angle relative to the horizontal. This motion moves material by accelerating the tray in an upward and forward stroke toward the discharge thereby increasing the normal force between the material and the tray and accelerating the material along with the tray. On the reverse stroke, the tray is accelerated down and rearwardly, away from the discharge thereby "unweighting" the material, reducing the normal force between the tray and the material to a lesser value, but not necessarily to zero. This movement allows the material to continue to travel in the feed direction, toward a discharge while the tray is moving in the opposite direction.
A requirement of this type of vibratory material handling system is that in order to maintain a consistent feed rate, the stroke amplitude of the tray must remain fairly constant. The stroke amplitude, however, on a resonant tuned machine is inherently unstable. The stroke will change with changes in material load, machine mass (including material adhering to the tray), and driving force. This is further compounded on closing-gap-magnetically-excited-machines by the fact that the driving forces is a function of the inverse of the air gap between the magnet and the armature which decreases as the stroke increases.
U.S. Pat. No. 5,293,287 describes an electromagnetic drive with a multipart spring system which uses sequential contacting between the moving mass and separate elastomeric springs to cause an increase in the spring force of the spring system as the stroke of the electromagnet increases. This system uses two side elastomeric springs which engage a moving plate initially, and a center elastomeric spring which engages the moving plate subsequently.
It would be desirable to increase the nonlinearity of a single elastomeric spring, so as to increase the total spring force non-linearly with increasing electromagnetic stroke, by modifying the elastomeric spring block itself, which is impacted by the moving mass.
It would be desirable to provide a resonant tuned vibration drive having an electromagnetic or mechanical driver which includes an elastomeric spring block arranged to be impacted by increasing stroke of the moving mass driven by the driver, wherein the spring block is modified to produce a non-linearly increasing spring rate in response to an increasing stroke of the moving mass. It would be desirable to provide such a device which would ensure that the stroke amplitude of the moving mass remains fairly constant to provide a consistent feed rate of bulk material transported by the moving mass.